You have wired new earths to the brakelights, but still no joy? Is the middle brakelight origional? You might want to disconnect that, as it could be adding to the complications. And then the cause of the light on the dash coming on..
So brake light switch is good, maybe take a feed off that middle brakelight to the outer brakelights (ie reverse what would normally be) ?
Possible fusebox problem? Take out relevent fuses and reseat them?
Dodgy relay? Don't think the brake lights use a relay, but worth pulling a relay at a time, and check the fault again?
I had the handbrake warning light showing - the (!) light, and it turned out to be caused by a shorted filament in the brakelight, but still worked! So car electrics are mysterious things.
Have you checked out the cars owners club? Most let you join for free, or browse and search?
Good luck!